In particular, rotorcraft are called on to operate in a wide variety of environments and under extreme conditions, which means that the turbine engines of such a rotorcraft need to be protected in order to withstand such conditions.
In order to feed air to a rotorcraft turbine engine, the turbine engine is fitted with an air inlet, the air inlet being provided with a duct that connects outside air to the turbine engine. Two types of air inlet can be distinguished, namely:                a dynamic air inlet fed with outside air under the effect both of the forward speed of the aircraft and of suction requested by the turbine engine; and        a static air inlet that is fed solely with air under the effect of the suction requested by the turbine engine.        
In order to prevent the turbine engine ingesting solid bodies that might damage it, e.g. birds, it is common practice to protect an air inlet with a grid. Said solid bodies are then blocked by the grid and do not run the risk of penetrating into the inside of the turbine engine.
Although effective, that solution presents a drawback under so-called “icing” flying conditions, and more particularly for dynamic air inlets. Under such flying conditions, icing occurs on the grid and thus obstructs the gaps through the grid in part or even completely.
Consequently, the air inlet can become partially or completely closed, which means that the power developed by the turbine engine will drop significantly, or even completely.
To remedy that, it is conceivable to make use of a grid that is overdimensioned. The dynamic air inlet has a flow area into which air penetrates dynamically into the dynamic air inlet, and the protected grid has first and second air flow sections, the first flow section facing the flow surface, unlike the second flow section. The grid can then be thought of as a kind of mushroom cap that covers the dynamic air inlet.
Thus, only the first flow section is liable to become iced. Under icing conditions, the second flow section then guarantees some minimum flow rate of air.
Such an overdimensioned grid is certainly ingenious. However it is found that dimensioning the grid is difficult. Furthermore, the overdimensioned grid possesses geometrical characteristics that are penalizing from an aerodynamic point of view, particularly since these characteristics are justified only when flying conditions are extreme.
Furthermore, aircraft pilots do not, a priori, have means enabling them to estimate the extent to which the first flow section of the overdimensioned grid has become clogged, and therefore have no means for deciding that a mission ought to be terminated because of advanced icing.
It should be observed that aircraft manufacturers have designed devices for protecting the air inlets of turbine engines so as to prevent any type of particle being ingested by such a turbine engine. For example, document FR 2 250 671 describes a multipurpose air inlet capable both of preventing particles being ingested by the turbine engine and of allowing flight under icing conditions without significant loss of performance from the turbine engine.
That multipurpose air inlet has a dynamic air inlet of annular shape forming a diverging section capable of being covered by a bullet-shaped member that moves axially. In addition the multipurpose inlet is provided with a cylindrical filtering inlet situated downstream from the dynamic inlet relative to the air flow direction, and provided on its outer periphery with a plurality of particle filters that act by inertia. When the anti-icing or anti-sand functions are activated, the bullet-shaped member blocks the dynamic air inlet so that all of the air ingested by the turbine engine passes through the filtering inlet, and more particularly through the particle filters.
These filters are provided with helical ramps that cause the air to swirl prior to penetrating into the inside of a tube having an outlet fitted with a separator that is concentric with the tube but of smaller diameter than the tube. Given the swirling movement generated by the helical ramp, a vortex phenomenon is observed at the inlet to each particle filter. Ice is then entrained towards the periphery of the vortex and ends up being deposited on the filtering inlet, more precisely on the outside edge of each particle filter, without blocking it. Thus, the filtering inlet prevents ice being absorbed in the turbine engine.
In its anti-sand mode of operation, the swirling movement of the air causes particles to be projected against the walls of the tube so that they cannot penetrate into the inside of the separator that serves to feed the turbine engine. Furthermore, a fan serves to extract the particles that have not passed through the separator on being projected against the walls of the tube.
Documents FR 1 585 516 and FR 1 548 724 both provide for implementing particle filters and describe a similar device.
Furthermore, in the state of the art that is remote from the invention, mention is made of document FR 2 924 471 that describes in particular compressible filter means, document EP 0 075 054 that uses a pivoting filter element, or indeed documents GB 577 799, WO 97/05942, and FR 2 538 453.